Secure System Design and Access Control

Nick FeamsterCS 6262

Spring 2009

2

What is Access Control?

• The process by which a computer system controls the interaction between users and system resources

• To implement a security policy, which may be determined by – organisational requirements– statutory requirements (medical records, for example)

• Policy requirements may include– confidentiality (restrictions on read access)– integrity (restrictions on write access)– availability

3

A Schematic View

• A user requests access (read, write, print, etc.) to a resource in the computer system

• The reference monitor– Establishes the validity of the request– Returns a decision either granting or denying access

to the user

Access Request

System Decision

Reference Monitor

4

Reference Monitor

5

Simple Analogies

• Consider a paper-based office in which certain documents should only be read by certain individuals

• We could implement security by – storing documents in filing cabinets – issuing keys to the relevant individuals for the

appropriate cabinets

6

Simple Analogy: Files

• The reference monitor is the set of (locked) filing cabinets

• An access request (an attempt to open a filing cabinet) is granted if the key fits the lock (and denied otherwise)

7

Simple Analogy: Guest List

• The reference monitor is the security guard + the guest list

• An access request is granted only if – a club-goer can prove their identity

(authentication) – she is on the guest list

8

Subjects and Objects

• Subject– Active entity in a computer system

• User, process, thread– We will assume that a subject is synonymous with

a user

• Object– Passive entity or resource in a computer system

• Files, directories, printers

9

Principals

• Principal and subject are both used to refer to the active entity in an access operation

• A principal is generally assumed to be an attribute or property associated with a subject– User ID– Public key– Process– Thread

• A subject may be represented by more than one principal

10

Access Operations

• An interaction between an object and a subject

• A subject may observe (read) an object– Information flows from object to subject

• A subject may alter (write to) an object– Information flows from subject to object

11

Access Control• given system has identified a user • determine what resources they can access• general model is that of access matrix with

– subject - active entity (user, process) – object - passive entity (file or resource) – access right – way object can be accessed

• can decompose by– columns as access control lists– rows as capability tickets

12

Acces Control Matrix

• Introduced by Lampson (1972) and extended by Harrison, Ruzzo and Ullman (1976-8)– Columns indexed by objects– Rows indexed by subjects– Matrix entries are (sets of) access operations– Foundation of many theoretical security models

trash a.out allfiles.txt

jason {r,w} {r,w,x} {r,w}

mick {r,x} {r}

ObjectsSubjects

13

Access Control Matrix

• A request can be regarded as a triple (s,o,a)– s is a subject– o is an object– a is an access operation

• A request is granted (by the reference monitor) if– a belongs to the access matrix entry

corresponding to subject s and object o

14

Access Control Matrix

• The request (jason, allfiles.txt, w) is granted• The request (mick, allfiles.txt, w) is denied

trash a.out allfiles.txt

jason {r,w} {r,w,x} {r,w}

mick {r,x} {r}

ObjectsSubjects

15

Disadvantages

• Abstract formulation of access control

• Not suitable for direct implementation– The matrix is likely to be extremely sparse and

therefore implementation is inefficient– Management of the matrix is likely to be extremely

difficult if there are 0000s of files and 00s of users (resulting in 000000s of matrix entries)

16

Access control lists

• An ACL corresponds to a column in the access control matrix• [a.out: (jason, {r,w,x}), (mick, {r,x})]• How would a reference monitor that uses ACLs check the validity of

the request (jason, a.out, r)?

trash a.out allfiles.txt

jason {r,w} {r,w,x} {r,w}

mick {r,x} {r}

ObjectsSubjects

17

Access Control Lists

• Access control lists focus on the objects– Typically implemented at operating system level– Windows NT uses ACLs

• Disadvantage– How can we check the access rights of a particular

subject efficiently (“before-the-act per-subject review”)?

18

Capability lists

• A capability list corresponds to a row in the access control matrix• [jason: (trash, {r,w}), (a.out, {r,w,x}), (allfiles.txt, {r,w})]• How would such a reference monitor check the validity of the

request (jason, a.out, r)?

trash a.out allfiles.txt

jason {r,w} {r,w,x} {r,w}

mick {r,x} {r}

ObjectsSubjects

19

Capability lists

• Capability lists focus on the subjects– Typically implemented in services and application

software– Database applications often use capability lists to

implement fine-grained access to tables and queries– Renewed interest in capability-based access control

for distributed systems

• Disdavantage– How can we check which subjects can access a given

object (“before-the-act per-object review”)?

20

Analogies Again

• An ACL is analogous to a guest list – the club is the object– the favoured clubbers appear on the list

• A capability list is analogous to the set of keys issued to a user (the subject) for unlocking the filing cabinets (the objects)

21

Trusted Computer Systems

• information security is increasingly important • have varying degrees of sensitivity of information

– cf military info classifications: confidential, secret etc

• subjects (people or programs) have varying rights of access to objects (information)

• known as multilevel security– subjects have maximum & current security level – objects have a fixed security level classification

• want to consider ways of increasing confidence in systems to enforce these rights

22

Control of Information in Computer Systems: Overview

• Focus of paper on multiple user computer system

• User authority– Who can do something to something– Who can see something

• “Privacy” social• Concern here is controlling access to data

(security)

23

Security Violations

• Unauthorized release of information• Unauthorized modification of information• Unauthorized denial of use of information

24

Definitions

• Protection – control access to information• Authentication – verify identity of user

25

Categories of Protection Schemes

• Unprotected– Typical batch system– Physical isolation (computer room)

• All or Nothing– User totally isolated in the system– No sharing of resources– Typical of early TS systems (Dartmouth BASIC)

26

Categories of Protection Schemes

• Controlled Sharing– OS puts limits on access– TOPS-10 file system w/ WRX control

• User-programmed Sharing Controls– Like OO files w/ access methods– User control access as he likes

27

Categories of Protection Schemes

• Putting strings on information– Trace or control information after released– File retains access even when others have it

• Overriding question: How controls can change over time– How is privilege changed?– Can access privilege be modified or revoked on

the fly?

28

Design Principles

• Since we can’t build software without flaws, we need ways to reduce number and severity of security flaws

• What follows are 10 Design Principles to apply when designing and creating protection mechanisms

• They were true in 1975 and remain relevant today

29

Design Principles

• 1. Economy of Mechanism– KISS Principle– Easier to implement– Allows total inspection of security mechanism

30

Design Principles

• 2. Fail-safe Defaults– Default case should be to exclude access

• Explicitly give right to access

– The reverse is risky• i.e. find reasons to exclude• You may not think of all reasons to

exclude

31

Design Principles

• 2. Fail-safe Defaults (cont.)– Easier to find and fix error that excludes

legitimate user• He will complain

– Won’t normally see error that includes illegitimate user

• He probably won’t complain

32

Design Principles

• 3. Complete Mediation– Check every access to objects for rights– All parts of the system must check– Don’t just open the door once and allow all

access after that• Authority may change• Access levels in security mechanism

may change over time

33

Design Principles

• 4. Open Design– Don’t try security by obfuscation or

ignorance• Not realistic that mechanism won’t be

discovered– Mechanism should be public– Use more easily protected keys or

passwords– Can be reviewed and evaluated without

compromising the system

34

Design Principles

• 5. Separation of Privilege– 2 keys are better than one– No one error or attack can cause security

violation– Similar to nuclear weapons authority

process– Not always convenient for user

35

Design Principles

• 6. Least Privilege– Programs and users should run with least

feasible privilege– Limits damage from error or attack– Minimize potential interaction among

privileged programs• Minimize unexpected use of privilege

36

Design Principles

• 6. Least Privilege (cont.)– Designing this way helps identify where

privilege transitions are• Identifies where firewalls should go

– Analogous to the “Need to Know” principle

37

Design Principles

• 7. Least Common Mechanism– Minimize mechanisms shared by all users

(e.g. shared variables)– Shared data can be compromised by one

user– If always shared, all users need to be

satisfied– Example – shared function should be run

in users space rather than as system procedure

38

Design Principles

• 8. Psychological Acceptability– Design for ease of use

• Then it will be used and not avoided or compromised for user’s convenience

– Model the mechanism so the user can easily understand

• Then will use as intended

39

Design Principles

• 9. Work Factor– Make it cost more to compromise security

than it is worth– Automation can make this principle difficult

to follow– Work factor may be difficult to calculate– Perhaps Public Key Encryption falls into

this category

40

Design Principles

• 10. Compromise Recording– Note all security breaches (and then

change plan?)• E.g. note each file access time

– Not very practical, since can’t tell what data has been taken or modified

41

Summary

• No security system is perfect – at best we minimize the vulnerabilities

• Technology may advance but engineering design principles which were good in 1975 remain relevant today